Protolytic Constants Research Articles

Formation of peptide disulfide bonds through a trans-dibromido-Pt(IV) complex oxidation reaction: Kinetic and mechanistic analyses

A trans-dibromido-Pt(IV) complex [PtBr2(en)2]Br2 was synthesized and evaluated for the synthesis of peptide disulfide bonds in this work. The reactions between the Pt(IV) complex and dicysteine-containing peptides were carried out in various aqueous solutions. As a result, excellent yields with fast reaction rates were achieved. Methionine residue was intact when the Pt(IV) complex reacted with a dicysteine and methionine-containing peptide. 3,6-Dioxa-1,8-octanedithiol (DODT) was selected as a model compound of the dicysteine-containing peptide. Kinetic studies for the reaction between the Pt(IV) complex and DODT were performed in different pH solutions. It is first-order both in [Pt(IV)] and in [DODT]. A reaction mechanism was proposed accordingly. The kinetic and mechanistic results demonstrated that the reaction rate for the formation of peptide disulfide bond via the Pt(IV) oxidation is increased with the increase of pH of the reaction medium. The protolytic constants of the two thiol groups in peptide also play a critical role in reaction rate. On the other hand, kinetic studies demonstrated that the Pt(IV) complex trans-[PtBr2(en)2]2+ can be used in an acidic medium for the purpose of synthesis of disulfide bonds.

Dissociation constants of protolytic dissociation of glutamyl-glutamic and glycyl-glutamic acids in aqueous solution at 298 K

Protolytic dissociation constants of stepwise dissociation of glutamyl-glutamic and glycyl-glutamic acids in aqueous solutions at 298.15 K at ionic strengths of 0.1, 0.5, and 1.0 M on the NaCl background were measured by potentiometric titration. The thermodynamic dissociation constants were calculated. Using the obtained and published data, we discuss the effect of additional carboxy groups in the side chains of the molecule on the protolytic dissociation constants of peptides.

Arsenate adsorption at the sediment–water interface: sorption experiments and modelling

Arsenate adsorption was studied in three clastic sediments, as a function of solution pH (4.0–9.0) and arsenate concentration. Using known mineral values, protolytic constants obtained from the literature and Kads values (obtained by fitting experimental adsorption data with empirical adsorption model), the constant capacitance surface complexation model was used to explain the adsorption behavior. The experimental and modelling approaches indicate that arsenate adsorption increases with increased pH, exhibiting a maximum adsorption value before decreasing at higher pH. Per unit mass, sample S3 (smectite–quartz/muscovite–illite sample) adsorbs more arsenate in the pH range 5–8.5, with 98% of sites occupied at pH 6. S1 and S2 have less adsorption capacity with maxima adsorption in the pH ranges of 6–8.5 and 4–6, respectively. The calculation of saturation indices by PHREEQC at different pH reveals that the solution was undersaturated with respect to aluminum arsenate (AlAsO42H2O), scorodite (FeAsO42H2O), brucite and silica, and supersaturated with respect to gibbsite, kaolinite, illite and montmorillonite (for S3 sample). Increased arsenate concentration (in isotherm experiments) may not produce new solid phases, such as AlAsO42H2O and/or FeAsO42H2O.

Insight into the degradation of a manganese(III)-citrate complex in aqueous solutions

AbstractKinetics of the electron transfer process between citrates and manganese(III) ions has been studied in acidic aqueous solutions. Acidification of the reaction mixture increased the reaction rate. The reaction is dependent on pH because there are two main protolytic forms of the Mn(III)-citrate complex in the studied pH range (4.5–6.5). Reduction potentials of Mn(III)/Mn(II) system in acidic and basic solutions as well as protolytic equilibria play a crucial role in understanding the pH profile of the studied system. The rate constants for Mn(III)citH and Mn(III)citH2+ species degradation processes are presented (citH3− and citH22− are trivalent and divalent anions of citric acid, citH4, respectively). Protolytic constant (expressed as pK′a) for Mn(III)citH protonation is estimated and discussed.

Multi-wavelength spectrophotometric determination of the protolytic constants of tetracycline hydrochloride in some nonaqueous-water mixed solvents: A solvatochromism study

Annihilation of the contribution of one chemical component from the original data matrix is a general method in rank annihilation factor analysis (RAFA). However, RAFA is not applicable for studying the protonation equilibria of multiprotic acids but in this study two-rank annihilation factor analysis (TRAFA) was used as an efficient chemometrics algorithm for determination of the protolytic constants (p K a ) of tetracycline hydrochloride (TCHC) in some nonaqueous-water mixed solvents such as acetonitrile (AN)–water and methanol (MeOH)–water from the spectral pH-absorbance data. The spectral data was obtained from spectrophotometric acid–base titrations of different solutions of TCHC at (25.0 ± 0.10) °C and an ionic strength of 0.10 M. In TRAFA algorithm the p K a values were obtained with relationship between residual standard deviation (R.S.D.) and hypothetical p K a values. In the case of TCHC, the spectra were divided in two consecutive subdivisions according to their pH range having two p K a and TRAFA was run twice. The validity of the obtained p K a values was checked with well-known chemometrics algorithms such as DATAN, EQUSPEC, SPECFIT/32 and SQUAD. The effects of changing solvent composition on the protolytic constants were explained by linear solvation energy relationships (LSER) utilizing solvatochromic parameters.

Modeling the Acid–Base Surface Properties of Aquatic Sediments

We propose a model that explains the acid–base surface properties of clastic sediments from two Argentinean reservoir lakes. The model uses potentiometric titration data sets and fixed parameters such as the apparent stability constants and reaction stoichiometries of acid–base equilibriums involving known mineral phases. The model considers that sediments act as a set of independent sorption surfaces, such as organic matter, clay silicate, and iron (hydr)oxides, thus the acid–base equilibrium and the correspondent protolytic constants are represented by a humic acid, a Na-illite, and a poor crystalline Fe-hydr(oxide). In agreement with experimental data, the model predicts that all sediment samples show a similar charging behavior, increasing the negative charge as the pH increases. The net charge of sediments is controlled by the presence of negatively charged minerals and/or organic matter coatings. This reveals the great influence of clays and organic matter functional groups on the acid–base surface properties of sediments, and consequently on the surface reactivity toward contaminant transport.

Effect of Ethanol on the Protolytic Properties of the Vitamins B Group

A multiwavelength spectrophotometric titration method was applied to study protolytic constants of four water-soluble vitamins, folic acid(vitamin B 9 or B 0 ), thiamine(vitamin B 1 ), riboflavin(vitamin B 2 ) and pyridoxal (vitamin B 6 ) in binary ethanol-water mixtures at 25°C and an ionic strength ofO.lM NaNO 3 . The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components has been calculated from the curve fitting of the pH-absorbance data with appropriate mass balance equations by an established factor analysis model. DATAN program was used for determination of acidity constant and SPECFIT program was used for calculation of standard deviations and partial correlation coefficients. A glass electrode calibration procedure based on the four parameter equation pH=α+SpcH+JH + [H + ]+ JOH - K w /[H + ] based on the Gran's plots was used to obtain pH-readings in the concentration scale (p c H). The effect of the solvent on the protolytic constants was discussed.

A new algorithm for the determination of protolytic constants from spectrophotometric data in multiwavelength mode: Calculations of acidity constants of 4-(2-pyridylazo)resorcinol (PAR) in mixed nonaqueous-water solvents

A new efficient, simple and versatile algorithm is presented for determination of the protolytic constants from spectrophotmetric data in multiwavelength mode based on the combining of hard and soft modeling. The algorithm was checked by determining the acidity constants of a triprotic acid from theoretical and real absorption–pH data. The real spectral data are obtained from photometric titration of different solutions of 4-(2-pyridylazo)resorcinol (PAR) by a standard base solution under an inert atmosphere. The algorithm starts the minimization process using an user supplied number of components and initial guesses of the unknown parameters and refined in a least squares manner. New algorithm is implemented in the new version of DATAN package (version 3.1). The validity of the obtained results was checked by some well known programs such as DATAN 2.1, SPECFIT/32, SQUAD, a modified version of difference spectra and a A–pH curve method. The comparison of the outputs of the DATAN 3.1 with the other programs reveals that there is a very good agreement between the obtained results and mentioned programs.

Multiwavelength spectrophotometric determination of protolytic constants of 4-(2-pyridylazo) resorcinol (PAR) in binary DMF–water mixtures

A multiwavelength spectrophotometric titration method was applied to study the protolytic constants of 4-(2-pyridylazo) resorcinol(PAR), in binary DMF + water mixtures. UV–vis absorption spectra of PAR solution were recorded in the course of pH-metric titration of acidic solutions of PAR with standard base solution. The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components have been calculated from the fitting of the pH-spectral titration data with appropriate mass balance equations by a home written program according to an established target factor analysis. To precise determination of number of absorptive components a recently developed statistical indicator function (IND function) was used. A glass electrode calibration procedure based on a four-parameter equation pH = α + S p c H + J H + [ H + ] + J O H − K w / [ H + ] based on the Gran's plots was used to obtain pH readings in the concentration scale (p cH). It has been observed that there is an inverse relationship between second and third protolytic constants and mole fraction of DMF. The effect of the solvent on the protolytic constants was discussed.

Reactions of excited states of phenoxazin-3-one dyes with amino acids

The interaction with amino acids of the excited states of the N-oxide resazurin and its deoxygenation product resorufin, has been studied in aqueous solution at pH 7.5. Steady-state and time-resolved studies show that the fluorescence is quenched by amino acids. Complexation of the dyes in the ground state with aromatic amino acids was also observed. The singlet quenching is attributed to electron transfer from the amino acids to the excited dye based on the dependence of the bimolecular rate constants with the ionization potential of quenchers. Flash photolysis experiments allowed determination of the quenching rate constants for the triplet deactivation of dyes by several amino acids, as well as the characterization of the transients formed in the process. These data show that the triplet is also deactivated by an electron transfer process. However, the deactivation of the N-oxide dye by tryptophan can be described by a hydrogen atom transfer. The protolytic dissociation constants of the dye radical ions are reported. The irradiation of rezasurin in the presence of amino acids leads to deoxygenation of the dye to give resorufin. This process involves the triplet excited state of resazurin and is efficient only in the presence of amino acids containing the -SH group.

Determination of Protolytic Constants by Trilinear Fluorescence Spectroscopy

Protolytic equilibria often have profound effects on chemical activity, since protolytic species usually behave quite differently. It is therefore important to characterize the protolytic properties of important chemicals. Here we present a new approach to study protolytic equilibria of fluorescent species that is extremely accurate and relies on minimum assumptions. We show that by measuring 2-dimensional excitation/emission scans of samples at different pH. the 3-dimensional experimental data set, I(lambda(ex), lambda(em), C(pH)), can be unambiguously decomposed into the spectral responses of the protolytic species present as well as their concentration. The approach is demonstrated on the protolytic equilibrium of fluorescein. Although the fluorescein monoanion cannot be obtained in pure form, the spectra and concentrations of both fluorescein species, as well as the protolytic constant, are determined with excellent accuracy. The proposed method is general and can be applied not only for studies of protolytic equilibria, but on any chemical equilibria and chemical reactions involving fluorescent species.

Influence of sodium dodecyl sulfate on the kinetics of complex formation between [PdCl(dien)] + and sulfur containing ligands l-cysteine and glutathione

The effect of sodium dodecyl sulfate (SDS) micelles on the kinetics of the complex formation between [PdCl(dien)] + and sulfur containing ligands l-cysteine and glutathione (GSH) was investigated by using the stopped-flow technique under pseudo-first order conditions (ligand in excess) in the acidity range from pH 1 to 6. The presence of anionic micelles induced the acceleration of the complex formation in the entire acidity range with the maxima corresponding to the first protolytic constant of the ligands. This effect was interpreted in terms of the attractive electrostatic interaction between reacting species and the micellar surface and their effective concentration in the vicinity of micelles. An increase of the ionic strength leads to a decrease of the rate of complex formation in the presence of anionic micelles due to competition of reactive species with cations originating from inert salt for the micellar surface. The calculation of activation parameters revealed that the entropy of activation is strongly negative in the presence and in the absence of micelles, which is compatible with an associative reaction mechanism.

Fluorescein uptake by a monocarboxylic acid transporter in human intestinal Caco-2 cells

The fluorescein transport characteristics of the human intestinal epithelial Caco-2 cell line were examined in monolayer cultures. The initial uptake rate was concentration-dependent and saturable; the Michealis constant and the maximum velocity were 0.40 mM and 1.32 nmol/min/mg protein, respectively. A protonophore, carbonyl cyanide m-chlorophenyl-hydrazone, reduced uptake significantly. The replacement of extracellular sodium ions by lithium ions did not alter the initial uptake rate. These facts imply that the transport is driven by a proton gradient. The initial uptake rate was strongly dependent upon extracellular pH, and the uptake was optimal at approximately pH 5.5. Based on the protolytic constants, the main species of fluorescein in the pH range of 5.5 to 6.0 was calculated to be a monoanion, suggesting that fluorescein was taken up by Caco-2 cells as a monocarboxylate. The following findings support this conclusion: the uptake was inhibited significantly by monocarboxylate compounds such as salicylate and pravastatin, but not by di- or tricarboxylic acids or by acidic amino acids. Furthermore, salicylate-preloaded cells showed remarkably enhanced uptake of fluorescein, indicating that monocarboxylates and fluorescein share a common transport carrier. The transporter has a wide spectrum of substrate recognition and seems likely to be different from MCT1.

Spectrophotometric study of the solubility and the protolytic properties of 1-(2-pyridylazo)-2-naphthol in different ethanol–water solutions

The solubility and the protolytic constants of 1-(2-pyridylazo)-2-naphthol (PAN) have been accurately determined at temperatures between 20.0 and 30.0°C in ethanol–water solutions with ethanol concentrations ranging from 10.0% to 95.0% (v/v). The measurement of the protolytic constants is based on the spectrophotometric determination of the concentrations of the corresponding conjugate acid–base pairs in ethanol–water solutions containing PAN and hydrochloric acid or sodium hydroxide. The traditionally used pH measurements in such studies in non-aqueous solutions are thus avoided. The solubility and the protolytic constants of PAN in pure water have been calculated by extrapolating the ethanol–water results to pure water where PAN is practically insoluble. The determination of the thermodynamic data mentioned above is necessary for elucidating the interactions of PAN with various metal ions in solutions and in cation-exchange membranes (e.g., Nafion ®) used as PAN-based optodes.

Protolytic constants of nizatidine, ranitidine and N, N′-dimethyl-2-nitro-1,1-ethenediamine; spectrophotometric and theoretical investigation

The prototropic exchange equilibria of two drugs, nizatidine (I) and ranitidine (II), and also of structurally related the N, N′-dimethyl-2-nitro-1,1-ethenediamine molecule (III) were investigated. From the changes in electronic spectra in media of various acidity several protonation constants were determined. For nizatidine p K values were −0.82, 1.95, and 6.67; for ranitidine p K values were 1.95 and 8.13; and for III was 2.60. The hydroxylation equilibrium constant in strongly alkaline media was determined too. Corresponding p K a values were 13.23 for I, 13.26 for II and 13.76 for III. Molecular orbital calculations of electronic spectra confirmed that p K 1.95 for I and II, and p K 2.60 for III, are associated with C-protonation of nitroethenediamine fragment, while all p K a values correspond to the addition of HO − anion at the same double bond.

Absorption and fluorescence properties of fluorescein

We have characterized the protolytic equilibria of fluorescein and determined the spectroscopic properties of its protolytic forms. The protolytic constants relating the chemical activities (which at low ionic strength equal concentrations) of the cation, neutral form, anion and dianion are p K 1 = 2.08, p K 2 = 4.31, and p K 3 = 6.43. All forms have rather high molar absorptivities being ε 437 FH 3 1 = 53 000, ε 434 FH 2 = 11 000, ε 453 FH− = 29 000 ( ε 472 FH = 29 000) and ε 490 F 2 = 76 900 M −1 cm −1 for the cation, neutral form, anion and dianion, respectively. The dianion has the most intense fluorescence with a quantum yield of 0.93 but also the anion shows considerable fluorescence with a quantum yield of 0.37. The neutral and cationic species are upon excitation converted into the anion and fluoresce with quantum yields of about 0.30 and 0.18, respectively.

Quantitative spectral analysis of multicomponent equilibria

A generalization is presented of a method for the determination of equilibrium constants for binary mixtures. It is shown that titrations involving several components can be quantitatively characterized in terms of component concentrations and spectral responses, even when none of the component spectra is known and the components cannot be obtained in pure form. The method is efficient even if the component spectra overlap extensively. It is applicable to all spectroscopic techniques that provide a linear response, opening up new possibilities to analyze complex chemical systems. The approach is illustrated by determining the absorption spectra and protolytic constants for the fluorescein protolytic equilibria.

Conformation-induced circular dichroism in hemoglobin of Chironomus th. thummi

The conformational isomers of Chironomus haemoglobin first postulated on the strength of the O2‐binding properties [24] and demonstrated by electron spin resonance spectroscopy [23] could now be confirmed by measurement of the circular dichroism.All the circular dichroism bands of the oxidized haemoglobin from Chironomus in the visible and Soret region are induced or influenced conformationally. The pH‐dependency of the extinction and the ellipticity of the individual bands follows a circuit process with four reaction steps, two of which describe the allosteric transformation and two the protolytic reaction of water at the 6th coordination point of the haem iron. With the aid of fit calculations we get the protolytic constants pK′2= 8.1 and pK”2= 8.3 and the allosteric constants pKa1= 7.3 and pKa2= 7.1.The existence of conformationally induced circular dichroism bands becomes obvious, if one compares the separated haemoglobins showing different degrees of allosterism. Thus for instance the individual haemoglobin IV, showing the smallest amplitude of the Bohr effect, is characterized by a minimal conformationally‐induced change of ellipticity. The individual haemoglobin VIII with the highest amplitude of Bohr effect shows the strongest conformationally‐induced circular dichroism bands.The pH‐dependency of the absorption spectra in the visible range displays further characteristics of conformational isomerism. The 4‐component system to be expected from the circuit process reveals at sufficient distance between the allosteric and protolytic pK values a system of two 2‐component mixtures, which can be transformed into each other depending on pH and thus produce “isosbestic triangles”.The helix content of the Chironomus haemoglobins is not influenced by the described change of conformation and amounts to 60 and 70% according to the individual haemoglobins.The results are discussed under the aspect of isomerism of conformations and allosterism controlled by the trans‐effect of the 5th ligand of the haem iron.

Studies on Derivatives of Malachite Green. Protolytic Equilibria and Reaction Rate Constants of o-Hydroxy Malachite Green.

Studies on Derivatives of Malachite Green. Protolytic Equilibria and Reaction Rate Constants of o-Hydroxy Malachite Green.

Studies on Aminonitrophenols. I. The Determination of the Protolytic Constants of 4-Amino-2-nitrophenol and 4-Amino-3-nitrophenol.

Studies on Aminonitrophenols. I. The Determination of the Protolytic Constants of 4-Amino-2-nitrophenol and 4-Amino-3-nitrophenol.